The present invention relates to the magnetic resonance arts. It finds particular application in conjunction with magnetic resonance imaging in open, or C-magnet, magnetic resonance imaging systems and will be described with particular reference thereto. However, it is to be appreciated that the present application will also find application in conjunction with other magnetic resonance imaging, functional magnetic resonance imaging, and spectroscopy systems in which the B0 primary magnetic field is orthogonal to the plane of the radio frequency coils.
Conventionally, magnetic resonance imaging procedures include disposing a subject in a substantially uniform, primary magnetic field B0. Dipoles in the subject preferentially align with the B0 field. Magnetic resonance is excited in the dipoles by transmitting radio frequency excitation signals into the examination region. Radio frequency signals emanating from the resonating dipoles are thereafter received by radio frequency coils and are processed to form readable images.
Most commonly, the B0 field is generated along the central bore of an annular magnet assembly, i.e., the B0 field aligns with the central axis of the patient. Cylindrical radio frequency and gradient magnetic field coils surround the bore. In order to improve the signal-to-noise ratio, quadrature surface coils have been utilized to examine a region of interest in quadrature, i.e., to receive signal components that are perpendicular to the coil and components that are parallel to the coil. See, for example, U.S. Pat. No. 4,918,388 of Mehdizadeh, which illustrates a loop coil and a flat Helmholtz coil, both of which receive resonance signals from the same region. The loop and flat Helmholtz coils are sensitive to orthogonal components of the magnetic resonance signal emanating from dipoles that are aligned with the horizontal magnetic field. When the output of one of the loop and flat Helmholtz coils is shifted by ninety degrees and the two signals are combined, the signal-to-noise ratio is improved by about the square root of two.
In order to examine larger regions of a subject disposed in the bore of a horizontal B0 field imager, surface coils consisting of a plurality of loop coils have also been used. See, for example U.S. Pat. No. 4,825,162 of Roemer and Edelstein. More specifically, a series of loop coils are partially overlapped in order to examine contiguous regions. As explained mathematically by Grover in xe2x80x9cInductance Calculationsxe2x80x9d (1946) and summarized in the Roemer and Edelstein patent, the mutual inductance between adjacent coils is minimized when the coils are positioned by a slight overlap. Although the use of overlapped loop coils with the induction minimized enabled a larger area to be examined, each coil was linear. That is, each coil was sensitive to only one component and not sensitive to the orthogonal component such that no quadrature detection was provided.
U.S. Pat. No. 4,721,913 of Hyde, et al. illustrates another surface coil technique for horizontal field magnets. A series of linear coils are arranged continuous to each other, but with each coil disposed ninety degrees out-of-phase with adjacent coils. Thus, each coil received a radio frequency magnetic resonance signal component that was orthogonal to its neighbors.
In U.S. Pat. No. 5,394,087 of Molyneaux, a loop and flat Helmholtz coil are superimposed to provide a flat quadrature coil. A plurality of these flat quadrature coils are partially overlapped to define a planar, quadrature coil array.
While the above-referenced surface coils are effective for horizontal B0 field magnetic resonance imaging equipment, all magnetic resonance imaging equipment does not employ a horizontal B0 field. C-magnet magnetic resonance imagers include a pair of parallel disposed pole pieces which are interconnected by a C or U-shaped iron element. The iron element may be a permanent magnet or can be electrically stimulated by encircling coils to a magnetic condition. Typically, the pole pieces are positioned horizontally such that a vertical field is created in between. Thus, in an annular bore magnetic field imager, the B0 field extends between the subject""s head and feet; whereas, in a C-shaped magnet the B0 magnetic field extends between a patient""s back and front. Due to the ninety degree rotation of the B0 field between the horizontal and vertical field magnets, quadrature surface coils such as illustrated in the above-referenced U.S. Pat. No. 5,394,087, when positioned along the subject in a vertical B0 field magnetic resonance imager, would not function in quadrature. They would loose sensitivity to one of their modes.
The present invention provides a new and improved radio frequency coil that provides quadrature reception/transmission in vertical B0 field magnets.
In accordance with one aspect of the present invention a magnetic resonance imaging apparatus is provided. The magnetic resonance imaging apparatus includes a main magnet assembly for generating a main magnetic field in a main magnetic field direction in an examination region, a gradient coil assembly for generating magnetic gradient fields in the main magnetic field within the examination region, a radio frequency transmit coil assembly for exciting resonance in selected dipoles within a subject disposed in the examination region such that the dipoles generate circularly polarized resonance signals at a characteristic resonance frequency, a radio frequency receive coil assembly for receiving the circularly polarized resonance signals generated by the dipoles, and a reconstruction processor for reconstructing the received signals into an image representation. The radio frequency receive coil assembly is disposed in the examination region substantially perpendicular to the main magnetic field direction and includes a substantially planar substrate and an array of quadrature coils disposed on the substrate. Each quadrature coil includes a first loop portion disposed on a first surface of the substrate and a second loop portion disposed on a second surface of the substrate.
In accordance with a more limited aspect of the present invention the second loop portion includes an offset from the first loop portion, the offset including a displacement in a direction perpendicular to the main magnetic field direction whereby mutual inductance between adjacent quadrature coils in the array is substantially eliminated.
In accordance with a more limited aspect of the present invention the main magnet assembly includes a vertical field magnet.
In accordance with a more limited aspect of the present invention the radio frequency coil assembly includes a surface coil.
In accordance with a more limited aspect of the present invention the first and second loop portions are hexagonally shaped.
In accordance with a more limited aspect of the present invention the first loop portion is capacitively coupled to the second loop portion to form a plurality of coils, each of the plurality of coils being sensitive to radio frequency signals perpendicular to the main magnetic field direction.
In accordance with a more limited aspect of the present invention the second loop portion includes a common ground loop.
In accordance with a more limited aspect of the present invention there is no overlap between adjacent quadrature coils in the array of quadrature coils.
In accordance with a more limited aspect of the present invention each quadrature coil further includes at least two takeoff points for taking signals off the quadrature coil and a phase shifter and combiner associated therewith for combining the signals in quadrature.
In accordance with another aspect of the present invention a method of magnetic resonance imaging is provided. The method includes the steps of generating a main magnetic field in a main direction in an examination region, generating magnetic field gradients in the main magnetic field, transmitting radio frequency signals into the examination region to excite selected dipoles in a subject disposed in the examination region such that the dipoles are circularly polarized in a plane perpendicular to the main direction, and receiving circularly polarized radio frequency signals from the excited dipoles using a receive coil assembly. The receive coil assembly includes an array of quadrature coils, each quadrature coil for receiving the radio frequency signals from the circularly polarized dipoles.
In accordance with a more limited aspect of the present invention the method further includes the steps of taking off signals from each quadrature coil from at least two takeoff points, phase shifting the signals from the at least two takeoff points, and combining the phase shifted signals in quadrature.
In accordance with a more limited aspect of the present invention the step of generating the main magnetic field includes generating a vertical magnetic field using an open magnet.
In accordance with a more limited aspect of the present invention the array of quadrature coils includes a two dimensional array and the array is disposed in the examination region substantially perpendicular to the main direction.
In accordance with a more limited aspect of the present invention each quadrature coil includes a first loop portion disposed on a first side of a substrate and a second loop portion disposed on a second side of a substrate, opposite the first side, such that there is substantially no mutual inductance between adjacent quadrature coils of the array.
In accordance with another aspect of the present invention, radio frequency receive coil assembly for receiving circularly polarized resonance signals in a magnetic resonance imaging system is provided. The radio frequency receive coil assembly includes a substantially planar substrate and an array of quadrature coils disposed on the substrate. Each quadrature coil includes a first loop portion disposed on a first surface of the substrate and a second loop portion disposed on a second surface of the substrate, the second surface being opposite the first surface.
In accordance with a more limited aspect of the present invention adjacent quadrature coils in the array are disposed relative to one another such that there is substantially no mutual inductance between the adjacent coils.
In accordance with a more limited aspect of the present invention there is no overlap between the adjacent coils.
In accordance with a more limited aspect of the present invention the assembly includes a surface coil.
In accordance with a more limited aspect of the present invention the array includes a two-dimensional array and the circularly polarized radio frequency signals include signals in a direction that is parallel to the array.
In accordance with a more limited aspect of the present invention the assembly further includes a radio frequency transmit coil.
One advantage of the present invention is that it provides a quadrature radio frequency receive coil that is substantially planar in a direction perpendicular to the main magnetic field of a magnetic resonance imaging system.
Another advantage of the present invention is that it provides a radio frequency coil that is oriented in a plane which is perpendicular to the main field of an open magnet and receives circularly polarized signals oriented in that plane.
Another advantage of the present invention is that the radio frequency coil is useful in phased array applications.
Another advantage of the present invention is that the radio frequency coil is useful in parallel spatial-encoding techniques such as SMASH, SENSE, and ASP.
Another advantage of the present invention is that the mutual inductance between coils of the radio frequency coil is eliminated.
Another advantage of the present invention is that the radio frequency coils are relatively thin and flat.
Still further advantages of the present invention will become apparent to those of ordinary skill in the art upon reading and understanding the following detailed description of the preferred embodiments.